Blue-streak cleaner wrasses live in harems. Dwelling amongst the coral reefs in the Indo-Pacific, the male carefully watches over his group of consorts, making sure that other males keep their distance. But the sole male cleaner fish also faces a threat from within his group: A female can usurp his spot. In fact, any of the females can change their sex – producing sperm instead of eggs within a few days – if they get large enough. To keep his ladies as ladies, the head honcho strictly enforces a diet.

Females that are close in size to the head male are punished more severely by him if they get selfish at the dinner table than small females, according to a recent study led by the Institute of Zoology in London and published in the Proceedings of The Royal Society B. The larger the female, the nearer she is to undergoing a sex change, so the male doesn’t like it when she eats too much. And he makes that dislike clear.

Cleaner fish eat the parasites and dead skin of larger fish. As they comb the big fish’s scales, they nibble away at the plentiful buffet of bugs and dandruff. It’s a win-win situation for all involved because while the cleaner fish get a meal, the larger fish gets, well, cleaned. It’s like going through a car wash: It comes out shiny. However, even though the cleaner fish scour the outside of the ‘client,’ they would really rather get a mouthful of the fish itself. It’s tastier and more nutritious. But if the cleaner bites the fish, then that fish swims away. So long buffet.

Most of the time, cleaner fish dine in pairs. The male and a chosen female feast on a fish simultaneously, which means they have to work together. If they don’t cooperate, and one bites the big fish, then they both lose out when the meal scrams. Yet the bad behavior is unfair. The biter gets a taste of the good stuff, while the one who accepted eating the skin isn’t even left with that.

Sometimes the cleaners dine out alone, though, and researchers noticed that the females cheat and bite the big fish more often when working solo compared to when they are cleaning with a male partner. That’s because the male punishes the female when she misbehaves. Should she bite the food source, the male vigorously chases her around.

Scientists wondered whether females who were punished learned their lesson for next time, so they tested it out in the lab. Instead of a real target fish, though, they made the client of Plexiglas and to the plastic they attached two food options: samples of prawn and samples of fish flake. The cleaners would rather eat the prawn, but if they nibbled at this delicacy, then the researchers gingerly whisked away the source of food to simulate the large fish leaving. For both the cleaners to benefit, they had to cooperate and eat the flake, a less tasty morsel.

After a female ate the prawn and the food source was taken away, the male would punish her aggressively, after which the researchers reintroduced the food source. They were testing whether the female would be more cooperative after being chastised. She was. Previous punishment meant she kept her errant biting in check the next time around.

But the experiments revealed something else as well: Males punished the females that were closer in size more severely than the smaller females. Because size is how the male asserts his dominance, a large female is a threat. It does more than merely challenge his rule, too, since she can physically become a male and take over the harem. Punishment serves a double purpose then. It enforces cooperation – selfishness ruins it for everyone – and keeps the male in control. He likes his ladies slim.

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